1. Field of the Invention
The present invention relates generally to diffractive light modulators, and more particularly, to a transmissive-diffractive light modulator in which a substrate having the same structure as that of a conventional diffractive light modulator has a light transmittance gate or is made of a transparent material, thus diffracting incident light while the incident light passes through the substrate.
2. Description of the Related Art
Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a great amount of data in real time, and studies have been conducted on the design and production of a binary phase only filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
Of them, the spatial light modulator is applied to optical memory, optical display device, printer, optical interconnection, and hologram fields, and studies have been conducted to develop a display device employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The light modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The light modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended on an upper part of a silicon substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on the oxide spacer layer 12 by a nitride frame 20.
In order to modulate light having a single wavelength of λo, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λo/4.
Limited by a vertical distance (d) between a reflective surface 22 of each ribbon 18 and a reflective surface of the substrate 16, a grating amplitude of the modulator 10 is controlled by applying a voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 formed on a lower side of the substrate 16 to act as a second electrode).
In an undeformed state of the light modulator with no voltage application, the grating amplitude is λo/2 while a total round-trip path difference between light beams reflected from the ribbon and substrate is λo. Thus, a phase of reflected light is reinforced.
Accordingly, in the undeformed state, the modulator 10 acts as a plane mirror when it reflects incident light. In FIG. 2, the reference numeral 20 denotes the incident light reflected by the modulator 10 in the undeformed state.
When a proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to move downward toward the surface of the substrate 16. At this time, the grating amplitude is changed to λo/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 using the interference. In FIG. 3, the reference numerals 28 and 30 denote light beams diffracted in +/− diffractive modes (D+1, D−1) in the deformed state, respectively.
However, the Bloom's light modulator adopts an electrostatic method to control a position of the micromirror, which has disadvantages in that an operating voltage is relatively high (usually, 20 V or so) and a correlation between the applied voltage and displacement is not linear, resulting in poor reliability in the course of controlling light.
To avoid the above disadvantages, there is suggested “a thin-film piezoelectric light modulator and a method of producing the same” as disclosed in Korean Pat. Application No. P2003-077389.
FIG. 4 is a cross-sectional view of a recess-type thin-film piezoelectric light modulator according to a conventional technology.
Referring to FIG. 4, the recess-type thin-film piezoelectric light modulator according to the conventional technology includes a silicon substrate 401 and elements 410.
In this regard, the elements 410, which have predetermined widths and are arranged at regular intervals, constitute the recess-type thin-film piezoelectric light modulator. Alternatively, the elements 410 having different widths may alternate to constitute the recess-type thin-film piezoelectric light modulator. As a further alternative, the elements 410 may be spaced apart from each other at regular intervals (each interval is almost the same as the width of each element 410), in which a micromirror layer formed on an upper side of the silicon substrate 401 reflects incident light to diffract it.
The silicon substrate 401 has a recess to provide an air space to each element 410, an insulating layer 402 is deposited on an upper surface of the substrate, and ends of the elements 410 are attached to upper sides of a wall of the recess.
The elements 410 each have a rod shape, and lower sides of ends of the elements are attached to the remaining upper side of the substrate 401 except for the recess so that the centers of the elements are spaced from the recess of the silicon substrate 401. Additionally, each element 410 includes a lower supporter 411 which has a vertically movable portion corresponding in position to the recess of the silicon substrate 401.
Furthermore, the element 410 is laminated on a left end of the lower supporter 411, and includes a lower electrode layer 412 for providing a piezoelectric voltage, a piezoelectric material layer 413 which is laminated on the lower electrode layer 412 and shrunken and expanded when a voltage is applied to both sides thereof to generate vertical actuating forces, and an upper electrode layer 414 which is laminated on the piezoelectric material layer 413 and provides a piezoelectric voltage to the piezoelectric material layer 413.
Furthermore, the element 410 is laminated on a right end of the lower supporter 411, and includes a lower electrode layer 412′ for providing a piezoelectric voltage, a piezoelectric material layer 413′ which is laminated on the lower electrode layer 412′ and shrunken and expanded when a voltage is applied to both sides thereof to generate vertical actuating forces, and an upper electrode layer 414′ which is laminated on the piezoelectric material layer 413′ and provides a piezoelectric voltage to the piezoelectric material layer 413′.
Additionally, Korean Pat. Application No. P2003-077389 describes an extrusion type as well as the recess type.
In the meantime, transmissive-spatial light modulators as well as conventional reflective spatial light modulators are representative examples of devices to spatially turn light ON/OFF. FIG. 4 is a view showing operation of a conventional transmissive-spatial light modulator.
As shown in FIG. 4, when natural light 501 enters into the conventional transmissive-spatial light modulator, the natural light 501 is polarized to form polarized light 503 having a directional character while passing through an incident-side polarizing plate 502.
The polarized light 503, which passed through the incident-side polarizing plate 502, rotates at an angle of 90° while passing through a liquid crystal part 504 of an OFF state. Thereafter, the polarized light 503 reaches an exit-side polarizing plate 506. At this time, the polarized light 503, which has been rotated at an angle of 90°, can pass through the exit-side polarizing plate 506, because the exit-side polarizing plate 506 crosses the incident-side polarizing plate 502.
However, in a liquid crystal part 505 of an ON state, light passing through the incident-side polarizing plate 502 directly reaches the exit-side polarizing plate 506 without being rotated at the angle of 90°. At this time, because the polarizing direction of the light reaching the exit-side polarizing plate 506 is perpendicular to the exit-side polarizing plate 506, the light cannot pass through the exit-side polarizing plate 506.
The above-mentioned principle is used in an LCD (liquid crystal display) in which transmissibility of light is controlled by applying voltage to a liquid crystal. In the LCD, a liquid crystal part 504, 505 is provided between an incident-side polarizing plate 502 and an exit-side polarizing plate 502.
Polarizing directions of the incident-side polarizing plate 502 and the exit-side polarizing plate 502 cross each other. The liquid crystal part 504, 505 changes the polarizing direction of light using applied voltage.
In light modulators having structures similar to the above-mentioned LCD, there are a light modulator which is turned ON/OFF by a signal formed on a Braun tube, and a reflective LCD in which a reflective film is formed on a surface of a liquid crystal part. These light modulators are also operated by the above-mentioned principle of polarizing incident light.